Magnesium oxide particle aggregate

ABSTRACT

The present invention is related to a magnesium oxide particle aggregate with the requirement of a first inflection point diameter is more than 0.30×10 −6  to 0.60×10 −6  m, a particle void volume is 0.50×10 −3  to 0.90×10 −3  m 3 ·kg −1 , and a micropore volume is 0.04×10 −3  to 0.11×10 −3  m 3 ·kg −1  in the cumulative intrusion volume curve of said particle by having controlled particle aggregation structure so that the solid phase-solid phase reaction between magnesium oxide and the SiO 2  film on the surface can be appropriately controlled.

This application is a 371 of PCT/JP01/09354 filed Oct. 25, 2001.

FIELD OF THE INVENTION

The present invention relates to a magnesium oxide particle aggregatehaving a controlled particle aggregation structure. More particularly,the present invention relates to a magnesium oxide particle aggregateused as an annealing separator to form a forsterite film which impartsexcellent insulation properties and magnetic properties to agrain-oriented magnetic steel sheet.

BACKGROUND ART

Grain-oriented magnetic steel sheets used in transformers or generatorsare generally produced by a process in which silicon steel containingabout 3% of Si is hot-rolled, subsequently cold-rolled so as to have afinal sheet thickness, and then subjected to decarburization annealing(primary recrystallization annealing), followed by finishing annealing.In this process, for imparting insulation properties to a magnetic steelsheet, after the decarburization annealing and before the finalfinishing annealing, a slurry containing magnesium oxide is applied to asurface of the steel sheet and then dried, and wound into a coil form.Si contained in the silicon steel sheet reacts with oxygen during thedecarburization annealing to form an SiO₂ film on the surface of thesteel sheet. SiO₂ in the film then reacts with magnesium oxide in theslurry during the finishing annealing to form a forsterite (Mg₂SiO₄)film having excellent insulation properties on the surface of the steelsheet. The forsterite film is considered to impart not only insulationproperties but also a tension to the surface thereof due to thedifference in the coefficient of thermal expansion between theforsterite film and the steel sheet, thus lowering core loss of thegrain-oriented magnetic steel sheet to improve the magnetic properties.

Therefore, the forsterite film plays an extremely important role in theproduction of grain-oriented magnetic steel sheets, and hence theproperties of magnesium oxide forming the forsterite film directlyaffect the magnetic properties thereof. For this reason, a number ofinventions have been made with respect to the magnesium oxide used as anannealing separator, especially having a controlled citric acid activity(CAA) between magnesium oxide particles and citric acid, and, forexample, Japanese Prov. Patent Publication Nos. 58331/1980, 33138/1994,and 158558/1999 have been disclosed.

However, CAA merely simulates empirically the reactivity in the solidphase-solid phase reaction between SiO₂ and magnesium oxide whosereaction actually proceeds on the surface of the magnetic steel sheet,based on the solid phase-liquid phase reaction between magnesium oxideand citric acid. Further, magnesium oxide is often present in the formof particle aggregate in which several powder particles bind togetherand agglomerate, and therefore CAA cannot appropriately evaluate theforsterite formation reaction.

On the other hand, Japanese Prov. Patent Publication No. 46259/1998discloses an invention made in respect of the fact that the film qualityvaries depending on the state of the pores in magnesium oxide. In thisinvention, the pore volume is restricted using a constant-capacity gasadsorption method, however, in the gas adsorption method, it determinesthe amount of gas molecules which adsorb onto the pore surfaces presenton the particle surfaces. Therefore, only pores as very small as, forexample, 0.1×10⁻⁶ m or less can be measured and thus, it is consideredthat this method is difficult to apply to the particle aggregationstructure having a size of about 1×10⁻⁵ to 1×10⁻⁶ m observed in theactual magnesium oxide particle aggregates, and hence cannotappropriately evaluate the forsterite formation reaction.

In view of this, the present inventors have found indices which canappropriately evaluate the forsterite formation reactivity of amagnesium oxide particle aggregate, and have completed an invention of amagnesium oxide particle aggregate having a particle aggregationstructure specified using the indices (Japanese Patent Application No.2000-132370). Namely, in the cumulative intrusion volume curve of themagnesium oxide particle aggregate, if these are controlled in a rangewhere a first inflection point diameter is 0.30×10⁻⁶ m or less, aninterparticle void volume is 1.40×10⁻³ to 2.20×10⁻³ m³·kg⁻¹, and aparticle void volume is 0.55×10⁻³ to 0.80×10⁻³ m³·kg⁻¹, it is possibleto form forsterite at a satisfactory rate on the surface of an magneticsteel sheet.

However, the present inventors have found that, with respect to therange considered to be unsuitable for the forsterite formation in theabove earlier patent application filed by the present inventors, forexample, a range in which the first inflection point diameter exceeds0.30×10⁻⁶ m, there is a possibility that excellent forsterite formationis achieved by further strictly controlling the particle aggregationstructure of the magnesium oxide particle aggregate.

An object of the present invention is to provide a magnesium oxideparticle aggregate having a particle aggregation structure furthercontrolled so that the forsterite formation rate can be appropriatelycontrolled.

In addition, another object of the present invention is to provide anannealing separator for grain-oriented magnetic steel sheet, using themagnesium oxide particle aggregate of the present invention, and toprovide a grain-oriented magnetic steel sheet obtainable by a treatmentusing the annealing separator for grain-oriented magnetic steel sheet ofthe present invention.

DISCLOSURE OF THE INVENTION

Namely, the present invention is a magnesium oxide particle aggregatecharacterized in that a first inflection point diameter is more than0.30×10⁻⁶ to 0.60×10⁻⁶ m, a particle void volume is 0.50×10⁻³ to0.90×10⁻³ m³·kg⁻¹, and a micropore volume is 0.04×10⁻³ to 0.11×10⁻³m³·kg⁻¹ in the cumulative intrusion volume curve of the particle.

In addition, the present invention is a magnesium oxide particleaggregate characterized in that an interparticle void volume in thecumulative intrusion volume curve of the particle is 0.80×10⁻³ to lessthan 1.40×10⁻³ m³·kg⁻¹, a particle void volume is 0.50×10⁻³ to 0.90×10⁻³m³·kg⁻¹, and a micropore volume is 0.04×10⁻³ to 0.11×10⁻³ m³·kg⁻¹ in thecumulative intrusion volume curve of the particle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows examples of cumulative intrusion volume curves of particleaggregates comprised mainly of magnesium oxide, determined from a poredistribution measurement by mercury porosimetry.

FIG. 2 shows a relationship between a forsterite formation rate, a firstinflection point diameter, and a particle void volume with respect tovarious magnesium oxide particle aggregates.

FIG. 3 shows a relationship between a forsterite formation rate, aninterparticle void volume, and a particle void volume with respect tovarious magnesium oxide particle aggregates.

FIG. 4 shows a relationship between a micropore volume and a forsteriteformation rate with respect to the particle aggregate having a firstinflection point diameter of more than 0.30×10⁻⁶ to 0.60×10⁻⁶ m and aparticle void volume of 0.50×10⁻³ to 0.90×10⁻³ m³·kg⁻¹.

FIG. 5 shows a relationship between a micropore volume and a forsteriteformation rate with respect to a particle aggregate having aninterparticle void volume of 0.80×10⁻³ to less than 1.40×10⁻³ m³·kg⁻¹and a particle void volume of 0.50×10⁻³ to 0.90×10⁻³ m³·kg⁻¹.

FIG. 6 shows temperature and time conditions suitable for controllingthe reaction so that the first inflection point diameter becomes morethan 0.30×10⁻⁶ to 0.60×10⁻⁶ m when magnesium hydroxide is prepared byreacting an aqueous solution of magnesium chloride with calciumhydroxide.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, a cumulative intrusion volume curve ofparticles refers to a curve which shows a relationship between a porediameter and a cumulative pore volume determined from a poredistribution measurement by mercury porosimetry, and FIG. 1 showsexamples of cumulative intrusion volume curves of two types of magnesiumoxide particle aggregates having different particle aggregationstructures. The first inflection point is the inflection point at thelargest pore diameter among inflection points at which the cumulativeintrusion volume curve suddenly rises, and it is indicated by a solidcircle in the figure. The first inflection point diameter refers to apore diameter at the first inflection point. The interparticle voidvolume refers to a cumulative pore volume at the first inflection point.The particle void volume is represented by a volume value obtained bysubtracting the cumulative pore volume at the first inflection pointfrom the total pore volume represented by the cumulative pore volume ata pore diameter of 0.003×10⁻⁶ m. The micropore volume refers to acumulative pore volume of micropores smaller than 0.05×10⁻⁶ m, and isrepresented by a volume value obtained by subtracting the cumulativepore volume at 0.05×10⁻⁶ m from the total pore volume and falls in theregion indicated by arrows near the right-hand ordinate in FIG. 1.

The present inventors have made studies on a solid phase-solid phasereaction between magnesium oxide and SiO₂, which reaction proceeds on asurface of a grain-oriented magnetic steel sheet, and, as a result, theyhave found that the first inflection point diameter in the cumulativeintrusion volume curve as determined from a pore distributionmeasurement by mercury porosimetry, a particle void volume, and ainterparticle void volume as well as a micropore volume can be used asindices for properly indicating the structure of a magnesium oxideparticle aggregate. Based on the above finding, these indices arecontrolled so as to fall in respective appropriate ranges to create amagnesium oxide particle aggregate which can appropriately control aforsterite formation on a surface of a grain-oriented magnetic steelsheet.

The pore distribution measurement by mercury porosimetry for obtainingindices indicating a particle aggregation structure was conducted by thefollowing method. In the measurements of pore distribution of poroussolid materials, the method of mercury porosimetry is well known as ananalysis method for obtaining pore distribution data of powder and dataabout a particle aggregation structure.

As a mercury porosimeter, AutoPore 9410, manufactured by MicromeriticsGbmH, is used. Measurement cell for powdery sample having a cellcapacity of 5×10⁻⁶ m³ and a stem capacity of 0.38×10⁻⁶ m³ is used. Asample to be measured is preliminarily treated using a 330 mesh standardsieve (JIS-R8801-87) so as to have substantially uniform particlediameters and then precisely weighed in the mass range of from 0.10×10⁻³to 0.13×10⁻³ kg, and placed in the measurement cell. The measurementcell is set in the porosimeter, and then the inside of the cell ismaintained in a reduced pressure state at 50 μHg (6.67 Pa) or less for20 minutes. Next, mercury is charged into the measurement cell until thepressure in the cell becomes 1.5 psia (10,342 Pa). Then, the mercury ispressed under a pressure in the range of from 2 psia (13,790 Pa) to60,000 psia (413.7 MPa) to measure pore distribution. As the mercury forthe measurement, a special grade mercury reagent having a purity of 99.5mass % or higher is used, and the density of the mercury used is13.5335×10³ kg·m⁻³.

The mercury pressing pressure is converted to a pore diameter using thefollowing formula (I)(Washburn's equation).D=−(1/P)·4γ·cos φ  (I)wherein D: pore diameter (m);

-   -   P: mercury pressing pressure (Pa);    -   γ: surface tension of mercury {485 dyne·cm⁻¹ (0.485 Pa·m)}; and    -   φ: contact angle of mercury (130°=2.26893 rad).

When pressing mercury into a particle aggregate, mercury firstpenetrates into the voids between the particles. In this instance, asthe mercury pressing pressure increases, that is, the pore diameterdetermined from the mercury pressing pressure decreases, the cumulativepore volume increases with a substantially constant gradient. After allvoids between the particles are filled with mercury, mercury startspenetrating into the voids in the particles. A great number of voidshaving substantially the same size are present in the particles, and thesum of the voids in the particles (the sum of the pore volumes) islarge. Therefore, when the penetration of mercury is changed fromthrough the voids between the particles to through the voids in theparticles, the cumulative pore volume drastically increases even as themercury pressing pressure slightly increases. This can be seen in thecumulative intrusion volume curves of FIG. 1.

A first inflection point diameter, a particle void volume, aninterparticle void volume, and a micropore volume are individuallydetermined from the cumulative intrusion volume curve as follows.

In the cumulative intrusion volume curves of FIG. 1, the cumulative porevolume on the ordinate indicates a cumulative value of the pore volumein the particle aggregate per unit mass of the sample determined fromlarger pore diameters successively. The inflection point is a point atwhich a cumulative intrusion volume curve suddenly rises. The number ofinflection point is not necessarily one, and, depending on the sample tobe measured, as can be seen in curve B in FIG. 1, there is the casewhere a plurality of inflection points are present, but the inflectionpoint at the largest pore diameter is taken as the first inflectionpoint. The first inflection point diameter is the pore diameter at thefirst inflection point. The interparticle void volume is a void volumebetween the aggregate particles, and it is represented by the cumulativepore volume at the first inflection point. The particle void volume is avoid volume which is present in the particles and smaller than thediameter of the aggregate particles, and it is represented by a volumevalue obtained by subtracting the cumulative pore volume at the firstinflection point from the total pore volume. The micropore volume isrepresented by a volume value obtained by subtracting the cumulativepore volume at a pore diameter of 0.05×10⁻⁶ m from the total porevolume. The total pore volume is a cumulative pore volume at a porediameter of 0.003×10⁻⁶ m. This is because the particle structure ischanged due to the mercury pressing pressure in the pore distributionmeasurement by mercury porosimetry, and therefore, the measurement errorcan be lowered by using the cumulative pore volume at the maximummercury pressing pressure as a total pore volume.

Next, magnesium oxide particle aggregates having different particleaggregation structures, in which a first inflection point diameter, aparticle void volume, a interparticle void volume, and a microporevolume in a cumulative intrusion volume curve are individuallydifferent, were prepared to examine the reaction rates of the solidphase reactions between the individual magnesium oxide particleaggregates and SiO₂. The results are shown in Table 1.

TABLE 1 First in- Forsterite flection Particle Interparticle Microporeforma- point void volume void volume volume tion diameter *10⁻³ *10⁻³*10⁻³ rate Unit *10⁻⁶ m m³ · kg⁻¹ m³ · kg⁻¹ m³ · kg⁻¹ % A 0.17 0.77 2.160.04 91.3 B 0.18 0.88 1.31 0.02 86.2 C 0.17 0.83 2.44 0.08 84.7 D 0.230.65 1.68 0.05 91.9 E 0.23 0.66 1.35 0.03 89.7 F 0.24 0.65 2.33 0.0186.5 G 0.21 0.85 1.81 0.03 89.5 H 0.22 0.58 1.43 0.01 90.5 I 0.25 0.562.12 0.01 90.1 J 0.22 0.52 1.59 0.01 86.8 K 0.27 0.74 1.46 0.05 91.8 L0.26 0.49 1.31 0.02 81.2 M 0.27 0.53 2.27 0.05 82.1 N 0.45 0.71 1.510.03 87.2 O 0.43 0.72 1.28 0.01 84.9 P 0.44 0.72 2.29 0.02 83.6 Q 0.430.91 1.62 0.02 81.0 R 0.42 0.48 1.45 0.01 78.8 S 0.59 0.52 2.25 0.0177.0 T 0.61 0.48 1.34 0 75.5 U 0.88 0.84 1.27 0 77.7 V 0.90 0.85 2.320.03 75.6 a 0.38 0.61 0.85 0.08 92.5 b 0.55 0.82 1.29 0.10 91.9 c 0.420.72 1.88 0.05 90.7 d 0.28 0.66 0.89 0.05 90.6 e 0.67 0.58 1.45 0.0687.6 f 0.95 0.77 1.89 0.10 79.3 g 0.41 0.54 1.24 0.02 85.6 h 0.35 0.721.42 0.13 87.3 i 0.45 0.45 1.37 0.07 85.4 j 0.39 0.94 1.28 0.09 86.1 k0.24 0.62 0.75 0.01 82.3 l 0.45 0.71 2.33 0.03 84.6 m 0.54 0.82 0.870.18 83.1

In the solid phase-solid phase reaction between magnesium oxide andSiO₂, these were directly reacted with each other to form forsterite.Namely, each magnesium oxide particle aggregate having an individualparticle aggregation structure and amorphous SiO₂ were mixed in a molarratio of 2:1 to form a mixture, and it was press-molded under a pressureof 50 MPa into pellets having a diameter of 15×10⁻³ m and a height of15×10⁻³ m. Then, the molded articles were calcined in a nitrogen gasatmosphere at 1,200° C. for 4 hours. With respect to the thus obtainedsintered product, an X-ray diffraction analysis was conducted toquantitatively determine a forsterite formation rate in the sinteredproduct.

With respect to the thus measured samples, a relationships between aforsterite formation rate, a first inflection point diameter, a particlevoid volume, a interparticle void volume, and a micropore volume in acumulative intrusion volume curve are shown in FIGS. 2 to 5. In FIGS. 2and 3, forsterite formation rate values are indicated by classifyinginto three levels, i.e., 90% or more, more than 80 to less than 90%, andless than 80%. In the figures, area a sectioned by a dotted linecorresponds to the range defined in the above-mentioned earlier patentapplication filed by the present inventors, and area b sectioned by asolid line corresponds to the range defined in the present invention.

The first inflection point diameter indicates the size of the largestparticle structure in the aggregate particles, and, the smaller thefirst inflection point diameter, the larger the number of contact pointsbetween the magnesium oxide particles and SiO₂, or the higher theactivity. Therefore, the first inflection point diameter is preferably0.30×10⁻⁶ m or less, which corresponds to the range in the earlierpatent application filed by the present inventors, and, when the firstinflection point diameter exceeds 0.30×10⁻⁶ m, the number of contactpoints between the magnesium oxide particles and SiO₂ generally lacks,making it difficult to form forsterite at a satisfactory rate. However,as can be seen in FIG. 2, there is a specific region such thatforsterite can be formed at a rate of 90% or higher at a firstinflection point diameter of 0.60×10⁻⁶ m or less even when the firstinflection point diameter exceeds 0.30×10⁻⁶ m. This region correspondsto the case where the particle void volume falls in the range of0.50×10⁻³ to 0.90×10⁻³ m³ kg⁻¹ (FIG. 2) and the micropore volume fallsin the range of 0.04×10⁻³ to 0.11×10⁻³ m³·kg⁻¹ (FIG. 4). The reason forthis is presumed that the highly active magnesium oxide particlesurfaces which are partially present on the particles make up for thelack of the contact points in this region. With respect to theforsterite formation, a value of 90% or more was taken as a satisfactoryvalue for the forsterite formation rate. When the forsterite formationrate can satisfy such a reference value, a forsterite film havingexcellent adhesion to the surface of a magnetic steel sheet can beformed.

On the other hand, the interparticle void volume is an index whichindirectly indicates the form of the aggregate particles, and, when theinterparticle void volume falls in an appropriate range, the contactpoints between the magnesium oxide particles and SiO₂ can beappropriately controlled. Therefore, when the interparticle void volumeis less than 1.40×10⁻³ m³·kg⁻¹, which corresponds to the range in theearlier patent application filed by the present inventors, the number ofcontact points between the magnesium oxide particles and SiO₂ generallylacks, making it difficult to form forsterite at a satisfactory rate.However, as can be seen in FIG. 3, there is a region such thatforsterite can be formed at a rate of 90% or higher at an interparticlevoid volume of 0.80×10⁻³ m³·kg¹ or more even when the interparticle voidvolume is less than 1.40×10⁻³ m³·kg⁻¹. This region corresponds to thecase where both the particle void volume and the micropore volume fallin the above respective ranges shown in FIGS. 3 and 5.

In other words, even when the magnesium oxide particle aggregate has afirst inflection point diameter and an interparticle void volume fallingin the respective ranges which are not suitable for the forsteriteformation in the earlier patent application filed by the presentinventors, forsterite is stably formed at a rate of 90% or higher aslong as both the particle void volume and the micropore volume fall inthe respective appropriate ranges.

Namely, the particle void volume is an index indicating the density ofthe aggregate particle. When the particle void volume is less than0.50×10⁻³ m³·kg⁻¹, the number of contact points lacks, and, when theparticle void volume exceeds 0.90×10⁻³ m³·kg⁻¹, the probability ofcontact between the magnesium oxide particle aggregates becomes toolarge, and hence magnesium oxide undergoes a reaction therebetweenbefore forming forsterite and is then inactivated. Thus, in any cases,forsterite is not formed at a satisfactory rate.

Further, the micropore volume is critical. The smaller the particles,the larger the curvature of the particle surface or the higher thesurface energy, then it makes the activity higher. Therefore, it isconsidered that the particle structure having micropores formed thereinis a highly reactive particle structure. However, when such a highlyactive particle structure has a micropore volume of more than 0.11×10⁻³m³·kg⁻¹, the probability of contact between the highly active particlestructures becomes too large, and hence the highly active particlestructures undergo a reaction therebetween before contributing to theformation of forsterite, so that forsterite is not formed at asatisfactory rate. On the other hand, when the highly active particlestructure has a micropore volume of less than 0.04×10⁻³ m³·kg⁻¹, thelack of the contact points cannot be made up for, so that forsteritecannot be formed at a satisfactory rate.

From the above results, when a requirement that the first inflectionpoint diameter be more than 0.30×10⁻⁶ to 0.60×10⁻⁶ m or theinterparticle void volume be 0.80×10⁻³ to less than 1.40×10⁻³ m³·kg⁻¹,the particle void volume be 0.50×10⁻³ to 0.90×10⁻³ m³·kg⁻¹, and themicropore volume be 0.04×10⁻³ to 0.11×10⁻³ m³ kg⁻¹ is satisfied,forsterite can be formed from the magnesium oxide particle aggregate andSiO₂ stably at a rate of 90% or higher.

Next, a magnesium oxide particle aggregate having a first inflectionpoint diameter, an interparticle void volume, a particle void volume,and a micropore volume each being in the above-mentioned range can beprepared as follows. It is noted that the preparation method describedbelow is merely one example, and a magnesium oxide particle aggregatehaving the particle aggregation structure defined in the presentinvention can be prepared by other methods.

The magnesium oxide particle aggregate can be prepared as follows. Forexample, calcium hydroxide is added to an aqueous solution of magnesiumchloride as a raw material to form magnesium hydroxide, and then themagnesium hydroxide is subjected to filtration by means of a filterpress, and dehydrated and dried, and then calcined using a rotary kiln,followed by grinding.

The magnesium oxide particle aggregate can be prepared by variousmethods, for example, a method in which an alkaline aqueous solution,such as an aqueous solution of calcium hydroxide, sodium hydroxide, orpotassium hydroxide, is reacted with a magnesium chloride-containingaqueous solution, such as bittern, brackish water, or sea water, toobtain magnesium hydroxide, and the magnesium hydroxide is calcined toobtain a magnesium oxide particle aggregate; a method in which magnesiteis calcined to obtain a magnesium oxide particle aggregate; a method(Aman method) in which a magnesium oxide particle aggregate is directlyobtained from a magnesium chloride-containing aqueous solution; and amethod in which magnesium oxide obtained by the above method issubjected to hydration to form magnesium hydroxide, followed bycalcination, to obtain a magnesium oxide particle aggregate.

The first inflection point diameter and particle void volume of themagnesium oxide particle aggregate can be adjusted by controlling theparticle structure of magnesium hydroxide which is a precursor ofmagnesium oxide, the interparticle void volume can be adjusted bycontrolling the conditions for grinding magnesium oxide obtained bycalcination, and the particle structure having micropores formed thereincan be obtained by mixing particle aggregates having differentcalcination degrees prepared by calcining magnesium hydroxide particleshaving a specific size or larger (BET specific surface area of 15×10³m²·kg⁻¹ or less).

The first inflection point diameter and particle void volume of themagnesium oxide particle aggregate are adjusted by controlling theparticle structure of magnesium hydroxide which is a precursor ofmagnesium oxide. Namely, a calcium hydroxide slurry is added to amagnesium chloride solution so that the resultant magnesium hydroxideconcentration becomes a predetermined value, and the resultant mixtureis stirred to effect a reaction at a predetermined temperature for apredetermined time, and then, the reaction mixture is subjected tofiltration by means of a filter press, and washed with water and driedto form magnesium hydroxide.

For adjusting the first inflection point diameter to be more than0.30×10⁻⁶ to 0.60×10⁻⁶ m, the reaction temperature and reaction time forthe magnesium hydroxide formation are controlled. Namely, as shown inFIG. 6, magnesium hydroxide is formed by a reaction under conditionssuch that the reaction temperature (T, ° C.) and the reaction time (t,hr) satisfy the relationship represented by the following formula (II).(1.033×10⁵) exp {(−8.5×10⁻²)·T}≦t≦(7.861×10⁶) exp {(−8.32×10⁻²)·T}  (II)Further, it is more preferred that the reaction temperature (T, ° C.)and the reaction time (t, hr) satisfy the relationship represented bythe following formula (III).(1.943×10⁶) exp {(−8.45×10⁻²)·T}≦t≦(2.180×10⁶) exp{(−8.35×10⁻²)·T}  (III)

The particle void volume is adjusted by controlling the magnesiumhydroxide concentration after the reaction. Namely, the ratio betweenthe magnesium chloride solution and the calcium hydroxide slurry mixedis adjusted so that the magnesium hydroxide concentration after thereaction becomes 0.2 to 4.5 mol·kg⁻¹, preferably 0.5 to 3 mol·kg⁻¹.

The micropore volume is adjusted by controlling the calcinationconditions (temperature×time) for the magnesium hydroxide formed, andmagnesium oxide particle aggregates having different calcination degreesare prepared and mixed with each other. In this case, the calcinationtemperature is 750 to 1,250° C., and the calcination time is 0.2 to 5hours. In the mixing, with respect to each of the magnesium oxideparticle aggregates obtained by calcination under specific calcinationconditions, a micropore volume is measured using a mercury porosimetrycurve and a mixing ratio is determined by making calculation, and then amixture having a specific micropore volume is obtained.

For obtaining a particle structure having micropores formed therein, itis required to form by calcination magnesium hydroxide particles havinga specific size or larger such that the BET specific surface area is15×10³ m²·kg⁻¹ or less. The crystal of magnesium hydroxide is oftrigonal crystal system, and generally has a hexagonal plate form. Inthe present invention, the magnesium hydroxide particles may be eithersingle crystalline or polycrystalline, and the form of the particles isnot limited to the hexagonal plate form, but the size of the magnesiumhydroxide particles indicated by the BET specific surface area isimportant. The reason for this resides in that, when the BET specificsurface area of the magnesium hydroxide particles exceeds 15×10³m²·kg⁻¹, it is difficult to obtain a particle structure havingmicropores required. Namely, when magnesium hydroxide changes tomagnesium oxide, a volume shrinkage of 50% or more occurs, andtherefore, in the smaller magnesium hydroxide particles, the magnesiumoxide particles formed moves due to deformation caused by the shrinkageto form relatively large magnesium oxide particles, thus making itdifficult to form small magnesium oxide particles needed to formmicropores in a specific range.

The interparticle void volume is adjusted by controlling the grinding ofthe calcined magnesium oxide. For example, when grinding using a hammermill-type grinder at a power of 5.5 kW having a classifier, the hammerrotational frequency is preferably 2,800 rpm or less.

As a grinder, a hammer mill-type grinder, a high-speed rotatingmill-type grinder, a jet mill-type grinder, a roller mill-type grinder,or a ball mill-type grinder can be used. The optimal conditions of thegrinder for obtaining the interparticle void volume which falls in therange defined in the present invention vary depending on the system andability (power) of the grinder used, but too strong grinding increasesthe interparticle void volume and too weak grinding lowers theinterparticle void volume. In the jet mill-type grinder in which theimpact energy applied during grinding is large, the impact energy maylower the particle void volume, and therefore the operation of thegrinder of this type needs to be controlled under conditions suitablefor the apparatus. Further, a classifier is not necessarily used, butthe use of a classifier makes it possible to control the grindingconditions more flexibly.

In the reaction of magnesium hydroxide, a flocculant can be added forpromoting the aggregation reaction, and a flocculation preventing agentcan be added for preventing the aggregation reaction from proceeding toan excess extent. Examples of flocculants include aluminum sulfate,polyaluminum chloride, iron sulfate, and polyacrylamide, and preferredare polyaluminum chloride and anionic polyacrylamide. The flocculant canbe added in an amount of 1 to 1,000 ppm, preferably 5 to 500 ppm, morepreferably 10 to 100 ppm, based on the total mass of the magnesiumchloride solution and the calcium hydroxide slurry. It is not preferredto add a flocculant in an excess amount since a particle aggregatehaving too high a density such that the particle void volume is lessthan 0.50×10⁻³ m³·kg⁻¹ is disadvantageously formed.

On the other hand, as a flocculation preventing agent, sodium silicate,sodium polyphosphate, sodium hexametaphosphate, a nonionic surfactant,or an anionic surfactant can be added, and preferred are sodiumsilicate, sodium hexametaphosphate, and nonionic surfactants. Theflocculation preventing agent can be added in an amount of 1 to 1,000ppm, preferably 5 to 500 ppm, more preferably 10 to 100 ppm, based onthe total mass of the magnesium chloride solution and the calciumhydroxide slurry. It is not preferred to add a flocculation preventingagent in an excess amount since a particle aggregate having such a lowdensity that the particle void volume is more than 0.90×10⁻³ m³·kg⁻¹ isdisadvantageously formed.

A calcium hydroxide slurry is added to a magnesium chloride solution,and then the stirring was conducted at a stirring rate of 350 to 450rpm. The stirring does not largely affect the particle structure, butthe interparticle void volume can be increased by stirring at a highspeed and at a high shear rate by means of, for example, a homogenizerduring the reaction or can be lowered by almost no stirring.

Next, using the thus obtained magnesium oxide, an annealing separatorfor grain-oriented magnetic steel sheet and a grain-oriented magneticsteel sheet are produced as follows.

A grain-oriented magnetic steel sheet is produced as follows: a siliconsteel slab having an Si content of 2.5 to 4.5% is hot-rolled and washedwith an acid, and then subjected to cold rolling or twice cold rollingincluding intermediate annealing so that the resultant sheet has apredetermined thickness. Then, the cold-rolled coil is subjected torecrystallization annealing, which also effects decarburization, in awet hydrogen gas atmosphere at 700 to 900° C. to form an oxide filmcomprised mainly of silica (SiO₂) on the surface of the steel sheet. Anaqueous slurry obtained by uniformly dispersing in water the magnesiumoxide particle aggregate having the particle aggregation structure ofthe present invention prepared by the above method is continuouslyapplied onto the resultant steel sheet using a roll coater or a spray,and dried at about 300° C. The thus treated steel sheet coil issubjected to final finishing annealing, for example, at 1,200° C. for 20hours to form forsterite (Mg₂SiO₄) on the surface of the steel sheet,and the forsterite imparts a tension to the surface of the steel sheetalong with the insulating film to improve the core loss ofgrain-oriented magnetic steel sheet.

As described in, for example, Japanese Prov. Patent Publication No.101059/1994, for facilitating the forsterite film formation, a knownreaction accelerator, inhibitor auxiliary, or tension-impartinginsulating film additive can be added to the annealing separator.

EXAMPLES

Next, the present invention will be described with reference to thefollowing Examples.

Example 1

A calcium hydroxide slurry was added to a magnesium chloride solutionhaving a concentration of 2.0 mol·kg⁻¹ so that the magnesium hydroxideconcentration after reaction became 1.2 mol·kg⁻¹, and the resultantmixture was subjected to reaction in an autoclave at 150° C. for 3 hoursto prepare magnesium hydroxide having a BET specific surface area of8.2×10³ m²·kg⁻¹. The magnesium hydroxide prepared was calcined using arotary kiln individually at temperatures of 800° C., 950° C., and 1,050°C. for one hour, and then ground by means of an impact grinder toproduce three types of magnesium oxide particle aggregates havingdifferent calcination degrees. Then, the three types of magnesium oxideparticle aggregates produced were mixed together in a mixing ratio of30:40:30 to obtain a magnesium oxide particle aggregate having aparticle aggregation form which falls in the range defined in thepresent invention.

Example 2

Magnesite was calcined using a rotary kiln at 1,100° C. for one hour toprepare magnesium oxide having a BET specific surface area of 5.2×10³m²·kg⁻¹. The magnesium oxide prepared was added to water so that theslurry concentration became 2 mol·kg⁻¹ to effect a reaction at 90° C.for 2 hours, preparing magnesium hydroxide having a BET specific surfacearea of 7.5×10³ m²·kg⁻¹. Then, the magnesium hydroxide prepared wascalcined using a rotary kiln at 980° C. individually for 0.2 hour, 0.5hour, 0.8 hour, and 2 hours, and then ground by means of an impactgrinder to produce magnesium oxide particle aggregates having differentcalcination degrees. Then, the four types of magnesium oxide particleaggregates produced were mixed together in a mixing ratio of 25:30:15:30to obtain a magnesium oxide particle aggregate in Example 2 having aparticle aggregation form which falls in the range defined in thepresent invention.

Example 3

A slaked lime slurry was added to bittern so that the magnesiumhydroxide concentration after reaction became 1.2 mol·kg⁻¹, and theresultant mixture was stirred at 600 rpm to effect a reaction at 80° C.for 2 hours. Then, the reaction mixture was subjected to filtration bymeans of a filter press, and washed with water and dried, and theresultant magnesium hydroxide was calcined using a rotary kiln at 900°C. for one hour to prepare magnesium oxide having a BET specific surfacearea of 20.6×10³ m²·kg⁻¹. The magnesium oxide prepared was added towater so that the slurry concentration became 3 mol·kg⁻¹, and thencalcium chloride was added thereto in an amount of 2 mol %, based on themole of the magnesium oxide, and the resultant mixture was subjected toreaction at 80° C. for 2 hours to prepare magnesium hydroxide having aBET specific surface area of 11.0×10³ m²·kg⁻¹. Next, the magnesiumhydroxide was calcined using a muffle furnace at a furnace temperatureof 1,200° C. individually for calcination times of 2 hours, 3 hours, and4 hours, and then ground by means of an impact grinder to producemagnesium oxide particle aggregates having different calcinationdegrees. Then, the three types of magnesium oxide particle aggregatesproduced were mixed together in a mixing ratio of 25:40:35 to obtain amagnesium oxide particle aggregate in Example 3 having a particleaggregation form which falls in the range defined in the presentinvention.

Comparative Example 1

A slaked lime slurry was added to bittern so that the magnesiumhydroxide concentration after reaction became 2 mol·kg⁻¹, and theresultant mixture was stirred at 600 rpm to effect a reaction at 80° C.for 2 hours. Then, the reaction mixture was subjected to filtration bymeans of a filter press, and washed with water and dried, and theresultant magnesium hydroxide was calcined using a rotary kiln at 890°C. for one hour to prepare a magnesium oxide particle aggregate. Then,the particle aggregate prepared was ground by means of an impact grinderto produce a magnesium oxide particle aggregate in Comparative Example 1having a specific particle aggregation structure in the earlier patentapplication filed by the present inventors (Japanese Patent ApplicationNo. 2000-132370).

Comparative Examples 2 to 4

The magnesium oxide particle aggregates obtained in Example 1 bycalcining magnesium hydroxide using a rotary kiln individually attemperatures of 800° C., 950° C., and 1,050° C. for one hour, and thengrinding the calcined product by means of an impact grinder were notmixed together but individually used.

Comparative Example 5

Bittern and slaked lime were reacted with each other at 40° C. for 10hours to form magnesium hydroxide, and then the magnesium hydroxide wascalcined by means of a rotary kiln at 1,050° C. The thus producedmagnesium oxide particles are not controlled with respect to theparticle aggregation structure as conducted in the present invention,and they are magnesium oxide for annealing separator used for generalmagnetic steel sheets.

Comparative Example 6

Slaked lime was added to sea water so that the magnesium hydroxideconcentration after reaction became 0.05 mol·kg⁻¹ to effect a reactionat 50° C. for 20 hours, thus forming magnesium hydroxide. 5 Hours beforecompletion of the reaction, anionic polyacrylamide was added in anamount of 200 ppm, and then the reaction mixture after completion of thereaction was subjected to filtration by means of a filter press anddried. Then, the resultant magnesium hydroxide was calcined by means ofa rotary kiln at 950° C. to prepare magnesium oxide particles. The thusprepared particles are not controlled with respect to the particleaggregation structure as conducted in the present invention, and theyare magnesium oxide used in an application other than annealingseparator.

Table 2 shows the measurement values for particle aggregation structuresof the particles or particle aggregates in Examples 1 to 3 andComparative Examples 1 to 6. As can be seen from this table, in each ofExamples 1 to 3 in which the particle aggregate was produced whilecontrolling the particle aggregation structure, the requirement of thepresent invention that the first inflection point diameter be more than0.30×10⁻⁶ to 0.60 ×10⁻⁶ m or the interparticle void volume be 0.80×10⁻³to less than 1.40×10⁻³ m³·kg⁻¹, the particle void volume be 0.50×10⁻³ to0.90×10⁻³ m³·kg⁻¹, and the micropore volume be 0.04×10⁻³ to 0.11×10⁻³m⁻³·kg⁻¹ is satisfied. In Comparative Example 1, the first inflectionpoint diameter is 0.30×10⁻⁶ m or less, the interparticle void volume is1.40×10⁻³ to 2.20×10⁻³ m³·kg⁻¹ and the particle void volume is 0.55×10to 0.80×10⁻³ m³·kg⁻¹, namely, the particle aggregate structure falls inthe range defined in the earlier patent application filed by the presentinventors. On the other hand, in each of Comparative Examples 2 to 4,the first inflection point diameter, the interparticle void volume, andthe particle void volume fall in the respective ranges defined in thepresent invention, but the micropore volume in Comparative Example 2exceeds the upper limit of the range defined in the present invention,and the micropore volume in each of Comparative Examples 3 and 4 is lessthan the lower limit. Further, in each of Comparative Examples 5 and 6,the particle aggregation structure is not controlled, and hence, theinterparticle void volume in Comparative Example 5 and the firstinflection point diameter in Comparative Example 6 fall outside of therespective ranges defined in the present invention, and almost nomicropores are present in the particles.

TABLE 2 Measurement values for particle aggregation structures Firstinflection point Interparticle Particle Micropore diameter void volumevoid volume volume Unit *10⁻⁶ m *10⁻³ m³ · kg⁻¹ *10⁻³ m³ · kg⁻¹ *10⁻³ m³· kg⁻¹ Example 1 0.38 1.26 0.74 0.09 Example 2 0.41 1.19 0.69 0.06Example 3 0.29 1.13 0.66 0.10 Com- 0.28 1.45 0.69 0.01 parative Example1 Com- 0.32 1.18 0.75 0.25 parative Example 2 Com- 0.35 1.25 0.77 0.02parative Example 3 Com- 0.55 1.38 0.72 0.01 parative Example 4 Com- 0.352.63 0.84 0.00 parative Example 5 Com- 1.17 0.52 0.62 0.00 parativeExample 6

Next, with respect to the above magnesium oxide particle aggregates orpowder particles, the behavior of formation of a forsterite film wasexamined. It is presumed that the formation of forsterite proceedsaccording to the solid phase reaction: 2MgO+SiO₂→Mg₂ SiO₄ Therefore, themagnesium oxide powder in each of Examples and Comparative Examples andSiO₂ were mixed in a molar ratio of 2:1 to form a mixture, and themixture was shaped under a pressure of 50 MPa to obtain a shaped articlehaving a diameter of 1.5×10⁻³ in and a height of 15×10⁻³ m, and then theshaped article was calcined in a nitrogen gas atmosphere at 1,2000° C.for 4 hours. This calcination temperature corresponds to the temperatureof the finishing annealing in which SiO₂ is reacted with a slurrycontaining magnesium oxide on the grain-oriented magnetic steel sheet.With respect to the thus obtained sintered product, an X-ray diffractionanalysis was conducted to quantitatively determine a forsterite(Mg₂SiO₄) formation rate. The results are shown in Table 3.

TABLE 3 Mg₂SiO₄ formation rate Mg₂SiO₄ formation rate/mass % Example 193.8 Example 2 91.8 Example 3 92.5 Comparative Example 1 90.6Comparative Example 2 85.6 Comparative Example 3 83.3 ComparativeExample 4 73.4 Comparative Example 5 77.5 Comparative Example 6 63.4

As can be seen from Table 3, in each of Examples 1 to 3, the forsteriteformation rate is higher than 90%. In Comparative Example 1 which fallsin the range in the earlier patent application filed by the presentinventors, the micropore volume is extremely small, but the forsteriteformation rate is higher than 90%. Thus, in this case, even when nomicropore is present, forsterite can be formed at a satisfactory rate.However, in each of Comparative Examples 2 to 6, the forsteriteformation rate is as unsatisfactory as less than 90%. Of these, theforsterite formation rate of the magnesium oxide in Comparative Example5 of an annealing separator in general use and that of the magnesiumoxide in Comparative Example 6 used in an application other than theannealing separator are very small.

Next, magnesium oxide was applied to an magnetic steel sheet to examinethe properties of a forsterite film. Steel to be examined is a steelsheet obtained by subjecting to hot rolling, washing with an acid, andcold rolling by a known method a silicon steel slab for grain-orientedmagnetic steel sheet, which slab comprises C: 0.058%; Si: 2.8%; Mn:0.06%; Al: 0.026%; S: 0.024%; N: 0.0050%, in terms of % by mass; and thebalance of unavoidable impurities and Fe, so that the final sheetthickness becomes 0.23 mm, and subjecting the resultant sheet todecarburization annealing in a wet atmosphere comprised of 25% ofnitrogen gas and 75% of hydrogen gas.

The magnesium oxide particle aggregates of the present invention and themagnesium oxide particles in Comparative Examples, each in a slurryform, were individually applied to the above steel sheet so that thedried coating weight became 12×10⁻³ kg·m⁻², and dried and then,subjected to final finishing annealing at 1,200° C. for 20 hours. Thestates of the individual forsterite films formed on the steel sheets areshown in Table 4.

TABLE 4 State of forsterite film formed State of glass film formedEvaluation Example 1 Uniform and ⊚ thick Example 2 Uniform and ⊚ thickExample 3 Uniform and ⊚ thick Comparative Example 1 Uniform and ⊚ thickComparative Example 2 Uniform and ◯ slightly thin Comparative Example 3Uniform and ◯ slightly thin Comparative Example 4 Nonuniform and Δslightly thin Comparative Example 5 Nonuniform and Δ slightly thinComparative Example 6 Nonuniform and X very thin

As can be seen from Table 4, the forsterite films formed from theparticle aggregates in Examples 1 to 3 and Comparative Example 1 arethose having a uniform and satisfactory thickness. Especially theforsterite films formed from the particle aggregates in Examples 1 to 3have been confirmed to have a large thickness and excellent adhesionproperties, as compared with the forsterite films formed from themagnesium oxide particles which are currently used as annealingseparators for high grade grain-oriented magnetic steel sheets.

INDUSTRIAL APPLICABILITY

As mentioned above, in the present invention, there can be providedmagnesium oxide having a particle aggregation structure which canadvantageously form forsterite. In addition, the magnesium oxideparticle aggregate of the present invention exhibits excellentforsterite formation, as compared with the magnesium oxide currentlyused as an annealing separator for grain-oriented magnetic steel sheets.Therefore, the grain-oriented magnetic steel sheet obtainable by atreatment using the magnesium oxide of the present invention hassatisfactory magnetic properties as a grain-oriented magnetic steelsheet. Further, the technical concept of the present invention can beapplied not only to the annealing separator but also to other solidphase reactions, for example, ceramic synthesis.

1. A magnesium oxide particle aggregate, characterized in that a firstinflection point diameter is more than 0.30×10⁻⁶ to 0.60×10⁻⁶ m, aparticle void volume is 0.50×10⁻³ to 0.90×10⁻³ m³·kg⁻¹, and a microporevolume is 0.04×10⁻³ to 0.11×10⁻³ m³·kg⁻¹ in the cumulative intrusionvolume curve of said particle aggregate.
 2. A magnesium oxide particleaggregate, characterized in that an interparticle void volume is0.80×10⁻³ to less than 1.40×10⁻³ m·kg⁻¹, a particle void volume is0.50×10⁻³ to 0.90×10⁻³ m³·kg⁻¹, and a micropore volume is 0.04×10⁻³ to0.11×10⁻³ m³·kg⁻¹ in the cumulative intrusion volume curve of saidparticle aggregate.